I work in the hospitality industry, not in Vegas. The answer all depends on certain factors.
You will have a better chance if you book the room directly through the Hotels website. If you use a discount website (Trivago Hotels.com etc) you may be paying less but you will get the rooms least desirable. Almost all Hotel websites in Vegas guarantee their rate is the lowest now.
It mostly depends on what the occupancy is.  Most rooms are preassigned a day or two before your arrival. If occupancy is high (less rooms available) chances are small. Also, talk to the valets and bellmen. They make there money on tips.
But Vegas is different. Money talks in that town

Yes , Just give it a shot

XoXo Sheila'

A lot of it depends on when you're checking in.  If you're arriving midweek during  the afternoon your chances are much better than if you're checking in at 8:00 pm on a Friday night.  The $20 trick tends to generally work pretty well in Vegas, generally with greater success at the mid-tier properties, but there are no absolutes.  It has worked for me dozens and dozens of times, so its definitely worth a shot.  I would think that if you are checking in at the Rio during non peak hours you have a good chance.  Don't feel awkward about doing it, the front desk personnel see this every day.    

I have a friend who visits Vegas frequently and he uses this trick, says it works about 80 percent of the time.  Often really nice upgrades too!

So, I do not splurge much on myself so...Given the proceding information I am wondering what would happen if I were to tip the check in clerk $100...  And I am arriving at the RIO at about 8pm on a Saturday

I've had good luck by being pleasant and asking for an upgrade nicely. Particularly if you can check in early in the day, because they'll probably have more comp upgrades available.

 
The game is this: You act nice, ask for an upgrade, and if they hesitate or you otherwise feel like they might decline, you slip them a 20. Worst case, you've just bought them a cheap lunch, which you can feel good about, best case, you get hooked up with a room upgrade and you're 20 bucks richer.

This usually works for the Lady's.  And if your going to the Rio the room are cheap why tip 100 when 100 extra can get you what you want for sure lol. Check in lines can be long and annoying at Rio because it's older but they have a fast check in process at looking things lined up enter your information scan I'd and cc  and it spits out your room keys with your paperwork. Done in 5m. I would just book your room upgrade online and fast pass your check in without speaking to anyone and start your vacay. I will note people get tips by the moments in Vegas and take it as a tip unless there in your world of your need and understanding and in a good mood lol. Basically make clear if tipping to expect something in return so they know what your trying to get and can do that if they can. It's never a guarantee but make sure it's out there what your tipping for hun. Don't be Shy!

you've already been comped for the room, by your host?

I've done the $20 trick many times in Vegas, with mixed results.  Depending on the hotel, ssometimes you'll get a very slight upgrade to the same room type with a better view.  

There is an entire website  dedicated to this, with success rates and people reporting back their results...

http://www.frontdesktip.com/las-vegas/hotels/